1. Field of the Invention
The present invention relates to a lithographic printing plate precursor, particularly, to a lithographic printing plate precursor containing an ultraviolet absorbing agent in its image-recording layer. More specifically, it relates to a lithographic printing plate precursor which is excellent in handling property under a mercury lamp and which is capable of forming an image by supplying printing ink and dampening water on a printing machine after imagewise exposure to remove the unexposed area.
2. Description of the Related Art
In general, a lithographic printing plate is composed of an oleophilic image area accepting ink and a hydrophilic non-image area accepting dampening water in the process of printing. Lithographic printing is a printing method utilizing the nature of water and oily ink to repel with each other and comprising rendering the oleophilic image area of the lithographic printing plate to an ink-receptive area and the hydrophilic non-image area thereof to a dampening water-receptive area (ink-unreceptive area), thereby making a difference in adherence of the ink on the surface of the lithographic printing plate, depositing the ink only to the image area, and then transferring the ink to a printing material, for example, paper.
In order to produce the lithographic printing plate, a lithographic printing plate precursor (PS plate) comprising a hydrophilic support having provided thereon an oleophilic photosensitive resin layer (image-recording layer) has heretofore been broadly used. Ordinarily, the lithographic printing plate is obtained by conducting plate making according to a method of exposing the lithographic printing plate precursor through an original, for example, a lith film, and then while leaving the image-recording layer corresponding to the image area, removing the unnecessary image-recording layer corresponding to the non-image area by dissolving with an alkaline developer or a developer containing an organic solvent thereby revealing the hydrophilic surface of support.
On the other hand, digitalized technique of electronically processing, accumulating and outputting image information using a computer has been popularized in recent years, and various new image-outputting systems responding to the digitalized technique have been put into practical use. Correspondingly, attention has been drawn to a computer-to-plate (CTP) technique of carrying digitalized image information on highly converging radiation, for example, a laser beam and conducting scanning exposure of a lithographic printing plate precursor with the radiation thereby directly preparing a lithographic printing plate without using a lith film. Thus, it is one of the important technical subjects to obtain a lithographic printing plate precursor adaptable to the technique described above.
Further, in the hitherto known plate making process of lithographic printing plate precursor, after exposure, the step of removing the unnecessary image-recording layer by dissolving, for example, with a developer is required. However, it is one of the subjects to save or simplify such an additional wet treatment described above. Particularly, since disposal of liquid wastes discharged accompanying the wet treatment has become a great concern throughout the field of industry in view of the consideration for global environment in recent years, the demand for the solution of the above-described subject has been increased more and more.
As one of simple plate making methods in response to the above-described requirement, a method referred to as on-press development has been proposed wherein a lithographic printing plate precursor having an image-recording layer capable of being removed in its unnecessary areas during a conventional printing process is used and after exposure, the unnecessary area of the image-recording layer is removed on a printing machine to prepare a lithographic printing plate.
Specific methods of the on-press development include, for example, a method of using a lithographic printing plate precursor having an image-recording layer that can be dissolved or dispersed in dampening water, an ink solvent or an emulsion of dampening water and ink, a method of mechanically removing an image-recording layer by contact with rollers or a blanket cylinder of a printing machine, and a method of lowering cohesion of an image-recording layer or adhesion between an image-recording layer and a support upon penetration of dampening water, ink solvent or the like and then mechanically removing the image-recording layer by contact with rollers or a blanket cylinder of a printing machine.
In the invention, unless otherwise indicated particularly, the term “development processing step” means a step of using an apparatus (ordinarily, an automatic developing machine) other than a printing machine and removing an unexposed area in an image-recording layer of a lithographic printing plate precursor upon contact with liquid (for example, ordinarily, an alkaline developer, a developer containing a surfactant or an aqueous solution containing a hydrophilic polymer) thereby revealing a hydrophilic surface of support. The term “on-press development” means a method or a step of removing an unexposed area in an image-recording layer of a lithographic printing plate precursor upon contact with liquid (ordinarily, printing ink and/or dampening water) by using a printing machine thereby revealing a hydrophilic surface of support.
In the simplification of plate making operation as described above, for example, in a dry processing system or a process-less system, since the image-recording layer after exposure is not yet fixed by development processing, still has sensitivity and has a risk of generating fog before printing, the image-recording layer capable of being handled in a bright room or under a yellow lump and a light source are preferably used. As such a light source, a semiconductor laser emitting an infrared ray having a wavelength of 760 to 1,200 nm and a solid laser, for example, YAG laser, are extremely useful because those lasers having a large output and a small size are inexpensively available.
The handling property of the lithographic printing plate precursor using such a light source in a bright room or under a yellow lump is fairly improved. However, there is a case of using a mercury lamp emitting light including an ultraviolet ray as the illumination lamp in the steps from plate making to printing and, when the lithographic printing plate precursor of on-press development type is allowed to stand for a long period of time under such a mercury lamp, a problem may arise sometimes in that fog is generated to deteriorate the removability of unexposed area at the on-press development.
In order to overcome the generation of fog, a method of incorporating an ultraviolet absorbing agent into an oxygen-blocking layer provided on an image-recording layer in an alkali-developing type CTP is proposed (see, for example, Patent Document 1). Although this method is an effective technique for a lithographic printing plate precursor which is subjected to an alkali-development processing, the effect becomes insufficient in some cases in the lithographic printing plate precursor of on-press development type because of the deterioration of on-press development property resulting from the incorporation of additive into the oxygen-blocking layer.
As an example of adding an ultraviolet absorbing agent to an image-recording layer, a lithographic printing plate precursor including microcapsules containing an ultraviolet absorbing agent in their walls (see, for example, Patent Document 2). This method is effective in an image-forming layer of heat fusion type or polarity conversion type in view of improvement in the stability of microcapsule. However, since the ultraviolet absorbing agent is not present uniformly throughout the image-recording layer, the method is hardly effective against the fog generation due to a mercury lamp and of little practical use in the lithographic printing plate precursor of on-press development type which is highly sensitive because of utilizing radical polymerization.    Patent Document 1: Japanese Patent 4279656    Patent Document 2: Japanese Patent 4222026